1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, in particular to a fixing device and an image forming apparatus employing a belt type fixing system using a belt as a fixing member, which is capable of accurately generating heat.
2. Description of the Related Art
As is well known, in an image forming apparatus employing an electro-photographic system, an electrostatic latent image is formed on a photoconductor as a latent image bearer and is rendered visible with toner to form a toner image. Subsequently, the toner image is transferred onto a recording medium, such as a recording sheet, etc., and is fixed thereon. The recording sheet is then outputted as a copy.
As a fixing system, a heat roller type is known in which a fixing roller and a pressing roller are arranged parallel and opposed to each other to sandwich a sheet therebetween and fix a toner image onto the sheet with heat provided from a heat source installed in the fixing roller and pressure applied from the pressing roller.
Also known is a belt-type fixing system, in which a fixing belt constructed of a good thermal conductor is employed as an alternative to the fixing roller. The belt-type system further includes a pressing roller, multiple rollers around which the belt is wound, and a heat source for heating the belt. An advantage of the belt-type fixing system is that a warm-up time needed to increase the temperature of the belt to a predetermined fixing temperature level can be shortened over that needed in the heat roller fixing system.
As an example of a fixing device employing such a fixing belt system, Japanese Patent Application Publication No. 11-002982 (JP-H11-002982-A) discloses a fixing system that includes a heating roller having a heater as a heat source, a fixing belt (i.e., a fixing member) that accommodates a fixing roller having a rubber surface within its loop, and a pressing roller (i.e., a pressing member) contacting the fixing belt outside the loop. In such a fixing system, when a recording sheet reaches the fixing device with a transferred toner image thereon and enters and passes through a nip formed between the pressing roller and the fixing roller (i.e., passes between the pressing roller and the fixing roller as conveyed by the fixing belt, the toner image is heated and pressed, and is accordingly fused in place on the recording sheet.
As an alternative to the above-described system, a film-type heating system is described in Japanese Patent Application Publication No. 04-44075 (JP-H04-44075-A), in which a fixed member sliding on an inner surface of a rotating member is employed, for example. Such a fixed member sliding on an inner surface of a rotating member is also employed in a pressing-type belt system as described in Japanese Patent Application Publication No. 10-213984 (JP-H10-213984-A).
However, a problem with the above-described fixing systems is that they require a long time to warm up due to bad durability, poor temperature stability, and large heat capacity, as well as slow temperature rising of the belt.
To solve the above-described problems, for example, Japanese Patent Application Publication No. 2007-334205 (JP-2007-334205-A) proposes a system in which a an endless belt is wound around a pipe state heat conductor made of metal (i.e., a metal pipe) to disperse heat uniformly over the entire inner surface of the endless belt, thereby stabilizing the temperature of the endless belt. As a development of the system of JP-2007-334205-A, for example, Japanese Patent Application Publication No. 2008-158482 (JP-2008-158482-A) proposes a technology utilizing a resistance heating element as a heat source for the endless belt.
In these systems, the endless belt moves or circulates in conjunction with rotation of the pressing roller opposed thereto and consecutively provides different belt portions thereof to a fixing position for fixing a toner image on a recording sheet to supply heat thereto.
Further, for example, Japanese Patent Application Publication No. 2008-216928 (JP-2008-216928-A) proposes a system that provides a small gap δ between the above-described resistance heating element and an inner surface of the endless belt (e.g., 0<δ≦1 mm, wherein δ represents the gap) to avoid contact between the heating element and the belt, so that the belt can circulate at high speed to facilitate uniform heating of the entire belt.
However, it turns out that a resistance heating element is such that its calorific value changes with temperature even at constant voltage, because the heating element's internal resistance changes. Consequently, temperature control of the fixing apparatus is degraded. As a result, in a configuration in which contact between a fixing belt as a fixing member and a resistance heating element as a heat generating member is maintained after a given image formation, the temperature of the resistance heating element fluctuates with an interval between the preceding image formation and the next image formation. As a result, even if the temperature of the fixing member is detected and a prescribed voltage is applied to the resistance heating element to control temperature, the temperature of the resistance heating element does not achieve the target temperature. Accordingly, the temperature of the fixing belt cannot ultimately be controlled as intended.